Honestly, if you want the short answer: HDPE poly lumber is the best all-around outdoor bench material for rainy climates because it won’t rot, rust, splinter, or warp. If your priority is a rot-proof bench that won’t develop rust streaks, this is the one I’d choose first. It’s made from recycled high-density polyethylene, so water can’t soak into it, and there’s no metal or organic fiber to corrode or decay. You just hose it off when it gets dirty.
That said, “best” depends on the look you want. If you prefer natural wood, high-quality teak is the classic rot-proof choice. Teak has natural oils and rubber that make it extremely resistant to moisture and decay, so it handles rainy seasons well. It won’t rust, obviously, and it won’t rot if you let water drain off it. The trade-off is it weathers to a silvery gray and benefits from an occasional cleaning or light sanding if you want to keep the original color.
For a metal look, powder-coated aluminum is my pick over steel or iron. Aluminum itself won’t rust, and a good powder coat keeps the frame protected from oxidation and scratches. Marine-grade stainless steel is also excellent, but it costs more, and lower grades can still show tea staining or pitting in coastal rain.
The materials I’d avoid for a truly low-maintenance rainy-season bench are unfinished softwoods, cheap tubular steel, and cast iron unless you love repainting and sealing. Also pay attention to the hardware: even a great HDPE or teak bench can develop rust streaks if the bolts are cheap. Look for stainless steel or coated fasteners.
So my practical ranking: choose HDPE poly lumber if you want zero fuss and strong weather resistance; choose teak if you want real wood with natural rot resistance and can accept the patina; choose powder-coated aluminum if you want a lightweight, modern rust-proof outdoor bench. Any of those will give you a weather resistant bench that shrugs off rainy seasons far better than ordinary wood or steel.